Paul Murdoch, who was sentenced to nine and a half years for possession of cocaine with intent to supply, talks to pupils at a school in Brent, north London. Photograph: David Levene for The Guardian

A voluble cry of disgust goes up as senior prison officer Mick Sherwood declares that the baggy boxer shorts dangling from his hand are “unisex prison underwear – three pairs a week and you don’t know who had them last week”. When former inmate Paul Murdoch adds that he worked in the prison laundry and washes were on such low temperatures that “skid marks” were common, the faces of the listening 11 and 12-year-olds are a picture of horror.

“Often it is the immediacy of the physical that first fires the students’ imagination and focuses them on the consequences of crime,” says Alison Zilberkweit, project co-ordinator of Your Life You Choose Brent, “but we know from the feedback that they take in the more serious issues too.” YLYC is a magistrate-led project, bringing together justices of the peace, the police, victim support, prison officers, and ex-offenders to provide a day of workshops for schoolchildren about the ease with which young people slip into crime and what that means for them, their families and their future.

“What do you want to be when you grow up?” demands Sherwood. “A doctor,” says one kid. “Can’t with a criminal record,” barks Sherwood; “Accountant,” calls another. “Nope,” says Sherwood; “Lawyer?” – “Forget it”; “Footballer?” – “Unlikely. The academies don’t want bad publicity.” And so on. Then he tells them prisoners get only two one-hour visiting slot per month: “Two hours a month to see family and friends.” There is a sharp collective intake of breath.

“YLYC is a key part of our strategy for getting young people to spot the warning signs and not get involved [in gangs and crime],” says Brent council’s head of community safety, Chris Williams. It is, he believes, the multi-agency approach and interactivity that make the project so successful.

YLYC Brent has so far been into nine schools, talking to 1,800 children, mostly aged 11 and 12. Next year the project will reach 2,600 pupils at 13 schools, with more schools expected to sign up. The scheme is a recent arrival in Brent but it has been going since 2008 in Ealing, where local police say it has contributed to falling youth crime. First time entrants to the youth criminal justice system in the borough have fallen from 346 a year to 117 – a drop of 73% in seven years.

YLYC also runs in three other London boroughs and in Sussex, and there are schemes in other areas also in the pipeline. The workshops vary according to local resources but all YLYC days begin with the same video.

More than 300 of Kingsbury’s year seven pupils watch the video: a young boy starts off as an innocent, doing well at school before encountering an older boy who offers him treats and requests simple errands. Slowly but surely he is drawn into the local gang. He makes a series of (mostly small) decisions that ultimately lead to his sister being threatened, his gran being beaten up and his arrest. It would be easy to make excuses for this boy. Nobody does. The message is clear: it’s his life, he chose.

The children are thoughtful as they split into smaller groups for the workshops. The police pursue the issue of choice, using an interactive video that allows them to decide what the characters will do (will the young man go to see what the fight is about, or go to his friend’s house?) and follows each set of choices through to its consequences. Two key messages are highlighted: the meaning of “joint enterprise” (and how easy it is to be guilty of it), and the dangers of carrying a knife.

The Your Life You Choose Brent project brings together the police, victim support, JPs, prison officers and former convicts for workshops in schools. Photograph: David Levene/The Guardian

“How many of you know someone who has carried a knife?” asks policewoman Donna Marshall. Most hands go up. Murdoch, brought up in Brent and now in his 40s, has his audience spellbound as he describes the decisions he made, from his schooldays onwards, that led to him losing his girlfriend and daughter and finding himself locked in a cell among a lot of “not very nice people” after being sentenced to nine and a half years for possession of cocaine with intent to supply.

You can see from the faces around the classroom that this is all going in.

“They take more notice of ‘real people’ than of parents and teachers,” says Sue Kayser, a magistrate and former teacher. “If we can just make them stop and think...”

YLYC costs less than keeping one youth in custody so, the organisers say, it is worth it both in human and financial terms if they divert just one child a year from crime. The scheme is staffed almost entirely by volunteers or people doing it as part of their job (such as the police officers). The ex-offenders are paid a small fee of £250-500, which Brent YLYC has had to charge to the schools this year, but next year this will be paid for out of £5,000 funding from a family charitable foundation, so it will be free to schools.

As the youngsters are leaving, I ask which things have stuck most in their minds. They talk about knives, responsible adults, awareness of consequences and how awful it would be not to see their families.

“I will think before I do things now,” says one young lad, in a typical response, “I will not carry a knife and if someone else is carrying one, I will talk to an adult ... I will do the right thing.”